Energy-sharing asymmetries in ionization by positron impact

The triply differential cross section of molecular hydrogen for ionization by 50 eV positrons has been determined, for the first time, for both the ejected electron in coincidence with the remnant ion and for the scattered projectile. Asymmetries in the energy sharing between the two light particles in the final state are observed, with the electron spectrum being shifted to significantly lower (and the scattered positron to correspondingly higher) energies than expected. A similar shape is observed in the case of the ejected electron spectrum from a helium target at the same excess energy

High-Resolution Measurements of e+ + H2O Total Cross Section

Using a purely positron beam, the total cross section of positrons scattering from H2O has been measured for the first time with a high angular discrimination (≃1°) against forward scattered projectiles. Results are presented in the energy range (10-300) eV. Significant deviations from previous measurements are found which are, if ascribed entirely to the angular acceptances of various experimental systems, in quantitative accord with ab initio theoretical predictions of the differential elastic scattering cross section

Magnetic field-free measurements of the total cross section for positrons scattering from helium and krypton

An electrostatic beam has been used to perform scattering measurements with an angular-discrimination of $\lesssim 2^\circ$. The total cross sections of positrons scattering from helium and krypton have been determined in the energy range (10–300) eV. This work was initially stimulated by the investigations of Nagumo et al (2011 J. Phys. Soc. Japan 80 064301), the first positron field-free measurements performed with a similarly high resolution, which found significant discrepancies at low energies with most other experiments and theories. The present results show good agreement with theories and several other measurements, even those characterized by a much poorer angular discrimination, implying a small contribution from particles elastically scattered at forward angles, as theoretically predicted for He but not for Kr

High-Resolution Measurements of e++ H2 O Total Cross Section

Using a purely electrostatic positron beam, the total cross section of positrons scattering from H2O has been measured for the first time with a high angular discrimination (≃1°) against forward scattered projectiles. Results are presented in the energy range (10-300) eV. Significant deviations from previous measurements are found which are, if ascribed entirely to the angular acceptances of various experimental systems, in quantitative accord with ab initio theoretical predictions of the differential elastic scattering cross section. © 2016 American Physical Society

Effect of positron-atom interactions on the annihilation gamma spectra of molecules

Calculations of gamma spectra for positron annihilation on a selection of molecules, including methane and its fluoro-substitutes, ethane, propane, butane and benzene are presented. The annihilation gamma spectra characterise the momentum distribution of the electron-positron pair at the instant of annihilation. The contribution to the gamma spectra from individual molecular orbitals is obtained from electron momentum densities calculated using modern computational quantum chemistry density functional theory tools. The calculation, in its simplest form, effectively treats the low-energy (thermalised, room-temperature) positron as a plane wave and gives annihilation gamma spectra that are about 40% broader than experiment, although the main chemical trends are reproduced. We show that this effective "narrowing" of the experimental spectra is due to the action of the molecular potential on the positron, chiefly, due to the positron repulsion from the nuclei. It leads to a suppression of the contribution of small positron-nuclear separations where the electron momentum is large. To investigate the effect of the nuclear repulsion, as well as that of short-range electron-positron and positron-molecule correlations, a linear combination of atomic orbital description of the molecular orbitals is employed. It facilitates the incorporation of correction factors which can be calculated from atomic many-body theory and account for the repulsion and correlations. Their inclusion in the calculation gives gamma spectrum linewidths that are in much better agreement with experiment. Furthermore, it is shown that the effective distortion of the electron momentum density, when it is observed through positron annihilation gamma spectra, can be approximated by a relatively simple scaling factor.Comment: 26 pages, 12 figure

Long Stent Implantation on the Left Anterior Descending Coronary Artery at a Follow-Up of More Than Five Years

Background: Stent implantation represents the standard of care in coronary intervention. While a short stent implanted on a focal lesion located on the left anterior descending artery (LAD) seems a reasonable alternative to an internal mammary implant, the same for long stents is still debated. Methods: We reported the long-term data of 531 consecutive patients who underwent Percutaneous Coronary Intervention (PCI) with long stents in two highly specialized centres. The main inclusion criteria were the implantation of stents longer than 30 mm on the LAD and a minimum follow-up (FU) of five years. The primary endpoint was mortality, and the secondary endpoints were any myocardial infarction (MI), target vessel and lesion revascularization (TVR and TLR, respectively), and stent thrombosis (ST) observed as definite, probable, or possible. Results: In this selected population with characteristics of complex PCI (99.1%), the long-term follow-up (mean 92.18 ± 35.5 months) estimates of all-cause death, cardiovascular death, and any myocardial infarction were 18.3%, 10.5%, and 9.3%, respectively. Both all-cause and cardiovascular deaths are significantly associated with three-vessel disease (HR 6.8; confidence of interval (CI) 95% 3.844–11.934; p &lt; 0.001, and HR 4.7; CI 95% 2.265–9.835; p &lt; 0.001, respectively). Target lesion (TLR) and target vessel revascularization (TVR) are associated with the presence of three-lesion disease on the LAD (HR 3.4; CI 95% 1.984–5.781; p &lt; 0.001; HR 3.9 CI 95% 2.323–6.442; p &lt; 0.001, respectively). Re-PCI for any cause occurred in 31.5% of patients and shows an increased risk for three-lesion stenting (HR 4.3; CI 95% 2.873–6.376; p &lt; 0.001) and the treatment of bifurcation with two stents (HR 1.6; 95% CI 1.051–2.414; p = 0.028). Stent thrombosis rate at the 5-year FU was 4.4% (1.3% definite; 0.9% probable; 2.1% possible), including a 1.7% rate of very-late thrombosis. The stent length superior to 40 mm was not associated with poor outcomes (all-cause death p = 0.349; cardiovascular death p = 0.855; MI p = 0.691; re-PCI p = 0.234; TLR p = 0.805; TVR p = 0.087; ST p = 0.189). Conclusion: At an FU of longer than five years, patients treated with stents longer than 30 mm in their LAD showed acceptable procedural results but poor outcomes.</p

Long Stent Implantation on the Left Anterior Descending Coronary Artery at a Follow-Up of More Than Five Years

Background: Stent implantation represents the standard of care in coronary intervention. While a short stent implanted on a focal lesion located on the left anterior descending artery (LAD) seems a reasonable alternative to an internal mammary implant, the same for long stents is still debated. Methods: We reported the long-term data of 531 consecutive patients who underwent Percutaneous Coronary Intervention (PCI) with long stents in two highly specialized centres. The main inclusion criteria were the implantation of stents longer than 30 mm on the LAD and a minimum follow-up (FU) of five years. The primary endpoint was mortality, and the secondary endpoints were any myocardial infarction (MI), target vessel and lesion revascularization (TVR and TLR, respectively), and stent thrombosis (ST) observed as definite, probable, or possible. Results: In this selected population with characteristics of complex PCI (99.1%), the long-term follow-up (mean 92.18 ± 35.5 months) estimates of all-cause death, cardiovascular death, and any myocardial infarction were 18.3%, 10.5%, and 9.3%, respectively. Both all-cause and cardiovascular deaths are significantly associated with three-vessel disease (HR 6.8; confidence of interval (CI) 95% 3.844–11.934; p &lt; 0.001, and HR 4.7; CI 95% 2.265–9.835; p &lt; 0.001, respectively). Target lesion (TLR) and target vessel revascularization (TVR) are associated with the presence of three-lesion disease on the LAD (HR 3.4; CI 95% 1.984–5.781; p &lt; 0.001; HR 3.9 CI 95% 2.323–6.442; p &lt; 0.001, respectively). Re-PCI for any cause occurred in 31.5% of patients and shows an increased risk for three-lesion stenting (HR 4.3; CI 95% 2.873–6.376; p &lt; 0.001) and the treatment of bifurcation with two stents (HR 1.6; 95% CI 1.051–2.414; p = 0.028). Stent thrombosis rate at the 5-year FU was 4.4% (1.3% definite; 0.9% probable; 2.1% possible), including a 1.7% rate of very-late thrombosis. The stent length superior to 40 mm was not associated with poor outcomes (all-cause death p = 0.349; cardiovascular death p = 0.855; MI p = 0.691; re-PCI p = 0.234; TLR p = 0.805; TVR p = 0.087; ST p = 0.189). Conclusion: At an FU of longer than five years, patients treated with stents longer than 30 mm in their LAD showed acceptable procedural results but poor outcomes.</p